Apparently, there are a few different crates you'll be able to fish up, and each one of them will have a different chance to drop the journal. The earliest you can obtain the journal will be at about 55 fishing, while the easiest you can obtain it appears to be from pools in Outland, that require a much higher fishing skill. If you need to powerlevel your fishing, you might want to start now, as it seems Blizzard wants the school tracking ability to be a reward for mid- to high-level fishermen. Which makes sense-- since it doesn't matter where you fish, the school tracking ability only helps when you're looking for a certain type of fish, and wouldn't really be necessary when just starting out as an angler.

Definitely sounds like a new bit of fun, and a fairly useful ability, for those who choose to level up fishing.